How do you calculate how smart an animal is? Sure, you could run rigorous tests and try to quantify the nature of "intelligence", but the quick and dirty trick is often just a comparison between brain and body mass. However, since this is a ratio, shouldn't evolution that changes body size also alter smarts? A group of researchers decided to find out.

The results appear in a new article in PNAS . The team of scientists tracked the evolution of brain and body size across hundreds of modern and extinct bats, carnivores, and primates — and what they found is that evolution seems to favor the body just as often as the brain.

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In some species, they found that brain mass evolution lagged behind body mass, whether increasing or decreasing. Bats, however, saw their body size decrease much faster than their brain, leading to small, maneuverable bats, able to easily forage cluttered environs. Generally speaking, it seems that evolutionary changes in brain and body size varied greatly from species to species.

In a release, lead author Jeroen Smaers said:

When using brain size relative to body size as a measure of intelligence, the assumption has always been that this measure is primarily driven by changes in brain size. It now appears that the relationship between changes in brain and body size in animals is more complex than has long been assumed. Changes in body size often occur independently of changes in brain size and vice versa. Moreover, the nature of these independent changes in brain and body size, are different in different groups of animals.

And in the paper, he and his colleagues wrote:

It is clear that comparative correlations involving relative brain size cannot be interpreted as selection on neuronal capacity alone. Relative brain size is the compromise of two traits taking potentially different evolutionary pathways involving different combinations of brain–body adaptations.

In other words, you can't just take the ratio of brain mass to body mass as an indicator of intelligence, as it's a complicated set of factors that lead towards animals being that size and shape. Some of it's evolutionary pressure towards intelligence, some of it's pressure towards changing body size. The reason a creature might be tiny with a large brain might be that it's an advantage to be small, not to be smart.